DELAVAN - When Delavan police officers arrived at his apartment with a search warrant in May 2012, the suspect knew to ask for a lawyer and keep his mouth shut.

This wasn’t his first time in the crosshairs of a reported sexual assault.

Just two years earlier, Delavan police arrested the same man in another sexual assault case. But that investigation fizzled when prosecutors declined to file charges.

It wouldn’t be the last time he eluded a courtroom.

Delavan police asked Walworth County prosecutors to consider charging the man this year after new DNA testing of a rape kit collected during their 2012 investigation. Again, prosecutors declined.

USA TODAY NETWORK-Wisconsin is not naming the man because he has not been arrested or charged with a crime. However, state Department of Justice prosecutors are reviewing the case and weighing whether to file charges themselves — just as they did earlier this year against a different man when Waupaca County prosecutors refused to charge him, despite a new DNA match from an old sexual assault case.

The state Department of Justice, under Attorney General Brad Schimel, has been overseeing a massive effort in recent years to test thousands of rape kits from old sexual assault cases. The evidence was never sent to crime labs for DNA analysis, in some cases sitting in police stations for decades.

As the effort has returned DNA matches with national databases, police officers across Wisconsin have begun revisiting their old cases and asking prosecutors to weigh charges. Three sexual assault cases from the effort have been filed in Winnebago, Waupaca and Rock counties. A fourth case is under review by Marinette County authorities.

In Walworth County, police referred charges after new DNA testing but District Attorney Zeke Wiedenfeld found the 2012 case didn’t meet his threshold for taking the suspect to court. Wiedenfeld said in an interview that he remained concerned about proving charges beyond a reasonable doubt.

"The issue in the charging decision is whether I could show there wasn't consent," he said.

Wiedenfeld declined to discuss details of the case. He said the charging decision could change if new evidence surfaces.

Delavan police records say the case involved a woman who reported drinking vodka with the suspect in his apartment, blacking out and then waking up naked to him assaulting her. The man was 36. Wiedenfeld said the woman was 30.

The woman agreed in 2012 to undergo an invasive medical exam at a hospital to collect evidence for DNA analysis, police records say. But that evidence, known as a rape kit, was left idle until the state's new testing effort.

Police records don't say why the rape kit wasn’t sent to crime labs for testing in 2012 even though a search warrant allowed investigators to collect the suspect's DNA to compare with other evidence. The records also don't say when the kit was tested.

Delavan Police Chief Tim O’Neill didn’t return messages seeking comment about the case. Wiedenfeld said the case was first referred to the district attorney's office for possible charges this year.

The sexual assault allegations that led to the suspect's arrest in 2010 were not immediately clear. USA TODAY NETWORK-Wisconsin has requested Delavan police reports from the case under state open records laws.

At the time of the 2010 arrest, Walworth County's district attorney was Phillip Koss, who has since become a circuit court judge. Koss said in an email that he couldn't recall the case.

Wiedenfeld said case records show the decision not to prosecute happened after assistant district attorney Diane Donohoo met with the victim, but he didn't elaborate further.

State authorities first discovered in 2014 that more than 6,000 rape kits were sitting untested in police stations and hospitals across Wisconsin. With $7.1 million in grants and the aid of private labs, the state has worked to test many of the kits and improve the criminal justice system's response to sexual assault.

As of Aug. 27, Department of Justice authorities reported obtaining confirmed DNA test results for more than 2,700 old rape kits. An additional 1,400 kits were still being tested or the results remained under technical review.

On a website tracking the project, the state says DNA analysis has produced 164 matches with national databases of offenders so far with 67 of the matches identifying a person who wasn't previously named as a suspect.

Wisconsin's response to old rape kits has become a prominent theme of Schimel’s re-election campaign this year. While the Republican has touted swiftly tackling the issue after taking office in 2015, his Democratic challenger, Josh Kaul, has criticized the state's response as too slow.